1. Field of the Invention
The present invention generally relates to liquid crystal display devices, and more particularly to a homeotropic alignment type liquid crystal display device which is suited for use in electronic equipments such as television sets, personal computers, monitoring units and projection type display units (projectors).
2. Description of the Related Art
As thin film transistor (TFT) type liquid crystal display (LCD) devices, there twisted nematic (TN) type LCD devices which employ a normally white mode. FIGS. 1A and 1B are diagrams for explaining the operating principle of a TN type LCD panel structure. As shown in FIGS. 1A and 1B, alignment layers are provided on corresponding transparent electrodes 12 and 13 which are formed on corresponding glass substrates, with a difference of 90xc2x0 in alignment directions, so as to sandwich TN liquid crystals therebetween. The liquid crystals contacting the alignment layer are arranged in the alignment direction of the alignment layer due to the nature of the liquid crystals. Hence, as shown in FIG. 1A, the liquid crystals are aligned such that the direction of the liquid crystal molecules is twisted by 90xc2x0. A pair of polarizing plates 11 and 15 are respectively arranged on respective sides of the transparent electrodes 12 and 13, in parallel with the alignment directions of the alignment layers.
When a non-polarized light 10 becomes incident to the LCD panel structure described above, the light passing through the polarizing plate 11 becomes a linearly polarized light before reaching the liquid crystals. Since the liquid crystal molecules are aligned with the 90xc2x0 twist, the incident light is also twisted by 90xc2x0 when passing through the liquid crystal molecules. As a result, the incident light passes through the lower polarizing plate 15. This state is referred to as a bright state.
Next, when a voltage is applied on the liquid crystal molecules by applying a voltage across the transparent electrodes 12 and 13 as shown in FIG. 1B, the liquid crystal molecules stand and the above described twist is eliminated. But since the alignment restricting force is strong at the alignment layer surface, the alignment direction of the liquid crystal molecules is still along the alignment layer. In such a state, the liquid crystal molecules are isotropic with respect to the passing light, and the polarizing direction of the linearly polarized light incident to the liquid crystals will not rotate. Accordingly, the linearly polarized light passing through the upper polarizing plate 11 cannot pass through the lower polarizing plate 15. This state is referred to as a dark state. Thereafter, when the voltage is no longer applied across the transparent electrodes 12 and 13, the liquid crystals return to the bright state due to the alignment restricting force.
On the other hand, as TFT type LCD devices, there are vertically aligned type LCD devices which employ a normally black mode. FIGS. 2A and 2B are diagrams for explaining the operating principle of a VA type LCD panel structure. As shown in FIG. 2A, alignment layers are provided on corresponding transparent electrodes 22 and 23 which are formed on corresponding glass substrates, with a difference of 180xc2x0 in alignment directions, so as to sandwich negative type liquid crystals (liquid crystals having negative dielectric constant anisotropy) therebetween. A pair of polarizing plates 21 and 25 are respectively arranged on respective sides of the transparent electrodes 22 and 23, with a 45xc2x0 difference to the alignment directions of the alignment layers. Axes of polarization of the two polarizing plates 21 and 25 are perpendicular to each other. As show in FIG. 2A, when no voltage is applied across the transparent electrodes 22 and 23 and thus no voltage is applied on the liquid crystal molecules, the liquid crystal molecules stand. However, because the alignment restricting force is strong at the alignment layer surface, the alignment direction of the liquid crystal molecules remain generally along the alignment layer (or slightly inclined to the alignment layer in some cases). In such a state, the liquid crystal molecules are isotropic with respect to the passing light. For this reason, when a non-polarized light 20 becomes incident to the LCD panel structure, the light passing through the polarizing plate 21 becomes a linearly polarized light before reaching the liquid crystals, but no change occurs in the polarized state of this linearly polarized light. Hence, the linearly polarized light passing through the upper polarizing plate 21 cannot pass through the lower polarizing plate 25, and thereby resulting in a dark state.
In addition, when a voltage is applied on the liquid crystals by applying a voltage across the transparent electrodes 22 and 23 as shown in FIG. 2B, the liquid crystals contacting the alignment layer are arranged along the alignment direction of the alignment layer due to the nature of the liquid crystals. Moreover, the liquid crystal molecules which are aligned cause other liquid crystal molecules to align thereto, and as a result, the liquid crystal molecules as a whole become aligned in one direction, that is, in an approximately horizontal direction with respect to the electrode surface. When the non-polarized light 20 becomes incident to the LCD panel structure, the light passing through the polarizing plate 21 becomes a linearly polarized light before reaching the liquid crystals. The light incident to the liquid crystal molecules can pass through the lower polarizing plate 25 due to a birefringence of the liquid crystals changing the polarized state, since a major axis direction of the liquid crystal molecules and the polarizing direction form an angle of 45xc2x0 thereby resulting in a bright state. Thereafter, when the voltage is no longer applied across the transparent electrodes 22 and 23, the liquid crystals return to the dark state because the liquid crystal molecules become approximately vertically aligned (form an approximate homeotropic alignment) with respect to the electrode surface (substrate surface) due to the alignment restricting force.
Compared to the TN type LCD device, the VA type LCD device has a high display contrast, a high response speed, and satisfactory viewing angle characteristics with respect to white and black displays.
Conventionally, various alignment processes have been proposed to align the liquid crystals of the LCD panel depending on the alignment layer. However, because of the weak alignment restricting force, the alignment was easily disturbed due to effects of an electrical field and the like, and there was a problem in that a disclination is easily generated. The disclination refers to a part where the alignment of the liquid crystals becomes discontinuous. In addition, when a rubbing process is carried out to align the liquid crystals, the alignment restricting force becomes relatively strong, but unevenness is easily generated in the alignment, and it was difficult to stably align the liquid crystals.
Accordingly, it is a general object of the present invention to provide a novel and useful liquid crystal display device in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a liquid crystal display device which can suppress the disclination and stably align the liquid crystals.
Still another object of the present invention is to provide a homeotropic alignment type liquid crystal display device comprising first and second substrates confronting each other, first and second bus lines arranged in mutually perpendicular directions on the second substrate, liquid crystals provided between the first and second substrates, and first, second and third projecting structures restricting alignment of the liquid crystals, where the first projecting structures have a sloping surface which is inclined with respect to the first substrate and are provided on the first substrate in parallel with the first bus lines, the second projecting structures have a sloping surface which is inclined with respect to the second substrate and are provided on the second substrate in parallel with the first bus lines, and the third projecting structures have a sloping surface which is inclined with respect to at least one of the first and second substrate and are provided on at least one of the first and second substrates with an arrangement different from the first and second projecting structures. According to the liquid crystal display device of the present invention, it is possible to suppress the disclination by the structural alignment provided by the projecting structures, thereby enabling a stable alignment of the liquid crystals.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.